The invention relates to a lithography system comprising an optical column wherein the interferometer is adapted for measuring a displacement, a moveable target carrier for displacing a target such as a wafer, and a differential interferometer, between a mirror provided on the optical column and a mirror provided on the target carrier. The present invention further relates to an interferometer module and method for measuring such a displacement.
A typical problem with interferometer based measurement systems is that small errors in the reflective surface of a mirror, for instance due to mirror unflatness and/or due to thermal expansion of the target carrier or optical column, result in displacement measurement errors due to Abbe and Cosine errors. Displacement errors negatively affect alignment precision needed for instance for stitching or overlay of a pattern.
Errors in Rz rotation of the optical column are especially a factor of importance in lithography system in which the orientation of the optical column, in particular of a projection lens optics thereof, substantially defines the orientation of the image projected thereby on a target. An example of lithography systems thus influenced is a multiple beam exposure system, in particular multiple charged particle beam exposure systems, in which each of the multiple beams is focused individually on a target by a projection lens array. In such a systems errors in Rx and/or Ry of the projection lens optics lead to focus errors when projecting an image onto a target exposure surface.
U.S. Pat. No. 7,224,466 provides a compact differential interferometer for measuring a displacement between a measurement mirror and a reference mirror along two axes. The interferometer uses shared measurement and reference beams that respectively reflect from measurement and reference reflectors before that shared beams are split into individual beams corresponding to the measurement axes of the interferometer. Though this interferometer emits three coplanar measurement beam spots on the measurement mirror and three corresponding coplanar reference beam spots on the reference mirror, movement along only the two measurement axes is measured.
U.S. Pat. No. 6,486,955 provides a system comprising a number of differential interferometers, some of which are used to track a displacement of a chuck mirror relative to a projection optics mirror along an X-direction as well as a rotation around an axis along a Z-direction. Additional interferometers are provided to measure displacement of the chuck mirror relative to the projection optics mirror along a Y-direction, tilt of the chuck relative to the projection optics around an axis along the X-direction and tilt of the chuck relative to the projection optics along the Y-direction respectively.
Thus, at least four interferometers are required to obtain information on displacement along the X-direction, the Y-direction and on tilt along the X-direction and Y-direction as well as a rotation along the Z-direction. This large number of interferometers increases complexity of the system and substantially increases downtime of the system needed for adjusting and/or replacing the interferometers.
It is an object of the present invention to provide a lithography system requiring fewer interferometers for obtaining information on relative displacement and rotation between a target carrier and an optics column.